A sensor element for determining the NOx, concentration in gas mixtures is known from German Published Patent Application No. 196 52 968 which is based on the interaction of two electrochemical pump cells. Two internal pump electrodes that are interconnected with a common external pump electrode arranged in a reference gas channel are located in a measuring gas space of the sensor element. The first of the two pump cells in the direction of inflow of the gas mixture causes oxygen to be transported from the measuring gas space into the reference gas channel. The internal pump electrode of this first oxygen-transporting pump cell is covered with a multilayered structure made of a mixed-conducting metal oxide layer and an electrically insulating aluminum oxide layer, which selectively removes the oxygen present in the gas mixture without changing the concentration of nitrogen oxides. These are then decomposed at the internal pump electrode of the second pump cell and the oxygen released in doing so is pumped off. The pump current of the second pump cell is used as a measure of the concentration of nitrogen oxides contained in the gas mixture.
A gas sensor for determining the NOx, concentration in gas mixtures is also known from European Published Patent Application No. 678 740. It includes two measuring gas spaces with one pump cell each, which are arranged adjacent to each other in a layer plane of a planar ceramic substrate. The measuring gas flows through a diffusion opening into the first measuring gas space in which a first pump cell is located. The first pump cell is used in the first measuring gas space to set a predetermined oxygen partial pressure by pumping oxygen in or out. A concentration cell also situated in the first measuring gas space makes it possible to maintain a constantly low oxygen partial pressure in the first measuring gas space by determining the electrical voltage (electromotive force) present at the electrodes of the concentration cell. Via an additional diffusion opening, the gas mixture set to a constant oxygen partial pressure enters the second measuring space. An additional pump cell is situated in the second measuring gas space. Its internal pump electrode is made of rhodium and it makes it possible to decompose nitrogen oxides to N2 and O2. The reduced oxygen arising at the internal pump electrode is pumped off via an applied pump voltage. The pump current of the second pump cell is proportional to the nitrogen oxide concentration of the gas mixture.
In both cases, a constantly low oxygen partial pressure of the gas mixture in the sensor element must be set in an elaborate manner before the nitrogen oxides can be determined with the internal pump electrodes used in them.